OBSERVING THE TRANSIT OF MERCURY, 2003 MAY 7 By: John E. Westfall, Coordinator, Mercury/Venus Transit Section, Association of Lunar and Planetary Observers (P.O. Box 2447, Antioch, CA 94531-2447, USA. e-mail: 73737.1102@compuserve.com )


The May 7, 2003, Transit of Mercury will be the first of sixteen times in the 21st century that a planet passes in front of the Sun as seen from the Earth. Fourteen of these events will be transits of Mercury, and two the rarer transits of Venus. The longitude visibility of the 2003 Mercury transit is similar to that of the 2004 Venus transit, so observers in the visibility zone of the two transits will have an opportunity, not only to observe the 2003 transit itself, but also to test their techniques and equipment in 2003 for the transit of Venus in the following year.

The sequence of events for all transits begins with the ingress phase, when the planet enters the disk of the Sun (ingress begins with the First Contact of the planet's limb with that of the Sun, and ends with Second Contact, when the planet is fully silhouetted on the Sun). The egress phase covers the movement of the planet off the face of the Sun (Third Contact marks the beginning of egress, while Fourth Contact is the end of egress). As seen by a geocentric observer, Mercury will enter the Sun's di k 15 degrees east of celestial north on May 7d 05h 13m UT, and then moves WSW across the disk, exiting 21 degrees north of west at 10h 32m. The entire event lasts about 5 hours 20 minutes and ingress and egress each last about 4.5 minutes. For actual observing locations, these times will vary at most by about 3 minutes from the geocentric times, while position angles will vary by less than one degree.

The 2003 transit is centered on the Eastern Hemisphere, meaning that the majority of the human race lives in its visibility zone:

-- The entire transit will be visible from Europe (except Portugal and western Spain), Africa (excluding the western coast and northwestern portion), and Asia (except for eastern Indonesia, the Philippines, Korea, Japan, and eastern Siberia).

-- Ingress, but not egress, can be seen from Alaska and the Canadian Yukon, eastern Indonesia, the Philippines, Korea, Japan, eastern Siberia, Australia and New Zealand.

-- Egress, but not ingress, will be seen from eastern Canada, the northeastern United States, the eastern Caribbean, and northeastern South America. In the eastern United States, the transit will end before sunset northeast of a line approximately from Charleston, SC to Dayton, OH to Marquette, MI. In eastern Canada, egress will be visible northeast of a line approximately from Thunder Bay, ONT to Coppermine, NWT.

An article describing this transit, as well as the Mercury transit of 2006 and Venus transits of 2004 and 2012, has appeared in the J.A.L.P.O., Vol. 45, Number 1 (Winter 2003). That paper contains a map of the worldwide visibility zone along with a table of 2003 transit viewing circumstances for selected cities throughout the zone of visibility.


Observing a transit is like observing sunspots. In fact, Mercury will look like a small, unusually dark, sunspot umbra. Note that the circular disk of the planet will be only 12.0 arc-seconds across. There is also a similarity to solar eclipse viewing because you have to do the observing at a particular time and may have to do considerable traveling, although your exact location is much less critical than for a total solar eclipse.

As with all solar observation, eye safety is a prime consideration. Eyepiece projection is relatively safe and can be used for general or group viewing, but remember that Mercury's disk will be quite tiny, with an apparent diameter only about 1/160 that of the Sun. Serious transit observing requires a telescope with a full-aperture solar filter, either aluminized glass or aluminized mylar. With so much light, a clock drive is not absolutely necessary for photography or electronic imaging, but it is v ry useful when tracking the tiny planet for more than a few minutes. With Mercury's small angular diameter, good seeing and optics, and high magnification are needed to observe the phenomena of this transit.

If, with the above filtration, the Sun's image is bright enough, an additional yellow or red filter is desirable in order to minimize the effect of solar limb darkening (Wratten #12, #15, #25, or even #29 Filters are suitable).

Finally, all contact times and times of images or photographs should be reported in UT (Universal Time) to a precision of 1 second. A GPS receiver will work for this, although short-wave radio time signals may be more convenient because they can be recorded on audiotape or videotape.

Because Mercury's disk will be so small, in order to find the planet you should take note of the predicted First Contact's position angle and carefully turn the telescope to that portion of the limb. Once Mercury's "bite" into the Sun becomes apparent, center the planet and then frequently reset the telescope to keep the planet centered as it crosses the face of the Sun. A sidereal drive rate is better to use than a solar one, but there will still be drift -- Mercury is moving westward in the sky fast r than the Sun is moving eastward!

Many observers will wish to watch the 2003 transit event simply because these events are relatively rare. However, there are several forms of observation of a transit that have scientific value as well.


There are several projects that one can choose from during this transit. They fall into the categories of: (1) observations that are personally interesting and challenging; and (2) those that, in addition, may actually extend our knowledge of Mercury, the Sun, and the phenomena that occur during transits.

General Observations

The "interesting and challenging" category of observations includes, but is not limited, to:

(1) Timings of Contacts II-IV. -- The best advice about what appearance to time was written over 40 years ago:

"Useful timings of contact I are not possible, as this stage is already past when the planet first becomes visible as a notch in the solar limb. For contact II, the time to record is the breaking of the 'black drop' that apparently connects the disk of Mercury with the solar edge. If no black drop is seen, the observer should note the instant when Mercury appears to be internally tangent to the sun's disk.

"At contact III, the events of II occur in reverse order. The phase to time is the breaking of the thread of light separating Mercury's black disk from the sun's edge. Contact IV is final disappearance of the notch in the solar limb, as Mercury moves off the sun." [Anon. 1960. "Observer's Page. November Transit of Mercury." Sky and Telescope, 20, 4 (Oct.): 214-215, 217.]

The contact timings should be recorded to a precision of 1 second, best done with voice comments on an audio tape with a radio time signal such as WWV in the background. If videotaping, the time signal can be recorded on the audio track. Video recording is very useful for contact timing because the videotape can be replayed at slow speed or even frame-by-frame. Later, an observer's contact timings may be compared with the predicted event times. With a transit of Mercury, agreement to within a few se onds is frequent.

(2) Observing Mercury off the Sun's limb. -- Sometimes Mercury has been reported as silhouetted against the inner corona or the chromosphere during a transit, either before Contact I or after Contact IV. Observers with Hydrogen-alpha filters should use them in this project to improve the image contrast and thus increase their chances for success. Indeed, the chromosphere may be wide enough in H-alpha to completely outline the disk of Mercury before Contact I and after Contact IV.

Unusual Optical Phenomena

There are several observing projects that are of scientific value because they may help us better to understand phenomena which, although perhaps illusory, have been repeatedly reported by observers of past transits. Visual drawings can be used here, although the supposed objectivity of photographs or video images may provide better data. Better yet would be sequences of CCD images, carefully calibrated with dark and background frames, which could give quantitative photometric information.

Light or dark aureoles around Mercury during transit. -- Such phenomena were regularly reported through the Nineteenth and early Twentieth Centuries, and more rarely thereafter. Since Mercury's atmosphere is optically negligible, these almost certainly are contrast effects. You may see such phenomena visually, as your eyes are subject to contrast effects; presumably photographic film or silicon chips will not record them, unless of course they are caused by the optics of one's telescope or projection ystem (i.e., by a Barlow lens or projection eyepiece). A possible "real" phenomenon that might conceivably be recorded would be a thin bright limb band outlining the portion of Mercury outside the photosphere, caused by sunlight scattered by dust electrostatically suspended above Mercury's surface.

Mercury's disk not completely black during transit. -- These phenomena range from a single light spot near the disk center, to one or more off-center spots, to the entire disk appearing other than black. Again, it is difficult to explain such phenomena other than as optical effects. If you see such an appearance visually, check if moving the telescope or changing the eyepiece affects the phenomenon. If it is caused by the telescopic or projection optics it might well be recorded photographically or e ectronically.

The Black Drop

The "Black Drop" might be called another "unusual optical phenomenon," except that it is reported for most transits, and should be expected for every transit. This term refers to a dark umbilicus that connects Mercury's limb with that of the Sun, both after theoretical Contact II and before Contact III, at times when the planet should appear completely within the Sun's disk.

Because Mercury's atmosphere is optically negligible, the separation of the Mercurian and solar limbs should be instantaneous were their images perfectly defined. This is of course never the case due to diffraction, atmospheric seeing, and the resolution limitations of film, CCD chips, or human eyes. Indeed, what is puzzling about the Black Drop effect is not its existence, but the fact that observers sometimes report that it is absent!

A.L.P.O. staff member Brian Cudnik is conducting a research program investigating the Black Drop, and is particularly interested in video recordings (preferably digital) of the periods near Second and Third Contact. If possible, use a filter of known transmission; narrow-band if you have enough light. Record all the supporting information described below and send a copy of your observations to Mr. Cudnik as well as the writer. (Brian Cudnik, 11851 Leaf Oak Drive, Houston, TX 77065 USA; e-mail: Brian Cudnik@pvamu.edu )


Supporting Information.--Observers will find that simply watching this rare phenomenon is of considerable personal interest; they may decide to also take notes, or to record it on film, videotape, or computer disk. However, their observations will not be of use or interest to others unless they are communicated, and are accompanied by the necessary background information as listed below.

Personal. -- Your full name, postal address, and e-mail address if you have one.

Type of Observation. -- Indicate all types of observation that you conducted; direct visual, eyepiece projection, photographic, video, CCD, and so forth.

Observing Conditions. - Set down the position of your observing site to an accuracy of 1 arc-minute (or 0.01 degree) in latitude and longitude, together with your elevation to the nearest 100 meters. Note the atmospheric seeing on the A.L.P.O. Scale (ranging from 0 for worst to 10 for best; if you are unfamiliar with this you may use the Antoniadi Scale), and atmospheric transparency (whether clear, hazy, cloudy, and so on). Note the beginning and ending of any breaks due to drifting clouds or for oth r reasons.

Instrumental. -- Describe your time source; telescope aperture, optical type, focal ratio and magnification if applicable; aperture stop if used; filter(s) employed; film type and exposure duration for photographs; video format ; exposure, and camera model and CCD chip type for CCD images; and any other instrumental data you believe are pertinent.

Observations. -- The writer welcomes all forms of observation of this event, including written notes, drawings; photographic prints, slides, or negatives; video tapes (NTSC VHS or SVHS format please); CCD images in the form of JPG or GIF files on IBM or MAC format diskettes or ISO 9660-format CD-ROMs. All drawings, photographs, and images need to have the direction of celestial north indicated as well as the UT to 1-second accuracy for all photographs or electronic images. We strongly recommend that you insert your name and the UT directly in the margin of any digital images.

Reporting Your Results. -- Please send copies of your observations to the writer, keeping the originals in your possession. However you report your results, the observations need to be accompanied by the supporting information listed above. Paper copy and computer diskettes can be sent to: A.L.P.O. Mercury/Venus Transit Section, P.O. Box 2447, Antioch, CA 94531-2447, U.S.A. Digital images or scanned non-digital images may be e-mailed in the form of TIFF or GIF files to: 73737.1102@compuserve.com. (P ease note that the pertinent background information needs to accompany any e-mailed data.)

Given sufficient observations received, the writer intends to prepare and publish in the J.A.L.P.O. a report on the 2003 transit. Finally, remember that, if you observe the 2003 transit of Mercury, you should be able to see the 2004 transit of Venus from the same place, but then you can benefit from your 2003 experience.

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